This invention relates to alpha alumina powders and specifically to alpha alumina powders with a number average particle width below 50 nanometers, (termed for brevity "nano-sized" particles) and to a method of making such powders. In discussing the "width" of such particles hereafter it is to be understood that, except where the context clearly indicates the contrary, it is intended to refer to the number average value of the largest dimension perpendicular to the longest dimension of a particle. The measurement technique is based on the use of a transmission electron microscope, (a JEOL 2000SX instrument).
Alpha alumina is the hardest and densest form of alumina and is formed by heating other forms of alumina or hydrated alumina at elevated temperatures. It is therefore the form of alumina that is best adapted to abrasive or ceramic applications.
Alpha alumina is conventionally formed by a fusion process in which an alumina hydrate is heated to above about 2000.degree. C. and then cooled and crushed. Heating at these high temperatures causes the crystals of alpha alumina to grow to several microns and to sinter together to produce an extremely hard material. The high density and the hardness of the alumina particles produced in this way make the crushing process very difficult. To get small particles, it is necessary to break the sinter bonds and, if even smaller particles are needed, perhaps of the order of a few microns or less in size, even to crush the primary crystals themselves. This is of course an extremely difficult task requiring much expenditure of energy. While the sinter bonds are very difficult to break, especially when sintering to essentially theoretical density has occurred, the fracture of the ultimate crystals themselves is even harder.
Recently the development of sol-gel, and particularly seeded sol-gel, processes have permitted the production of alumina with a microcrystalline structure in which the size of the ultimate crystals, (often called microcrystallites), is of the order of 0.1 micron. Such seeded processes incorporate seed particles that are capable of nucleating the conversion of boehmite, (alpha alumina monohydrate), to the alpha alumina phase at relatively low temperatures. The nature of the seed particle in terms of its crystal shape and lattice dimensions should be as close as possible to that of the target material for the nucleation to be efficient so that the logical choice is alpha alumina itself.
Virtually as soon as the alpha phase is generated, in the form of particles comprising microcrystallites of alpha alumina less than one micron in size, there is a tendency for the particles to sinter together where they contact one another. This tendency accelerates with increasing temperature. Keeping the temperature of formation of the alpha phase low therefore minimizes the degree to which the particles are sintered together and thus makes crushing to the ultimate particles size somewhat easier.
In U.S. Pat. No. 4,657,754, Bauer et al. teach firing a dried seeded sol-gel alumina to convert at least a portion to the alpha phase and then crushing the dried product to a powder of alpha particles, taking care not to cause excessive sintering or particle growth during the firing. This ensures that little sintering will have taken place. Thus the crushing will need to break only a few sinter bonds and no ultimate particles. Firing to complete the conversion can then be undertaken with the product already in its powder form. This is still a difficult and expensive operation however and limited essentially by the size of the ultimate particles of alpha alumina in the product, (100 nm).
Finer crystallites can of course be obtained by the use of finer seed particles. If the size of the seed particles is of the order of 0.1 micron then the final product obtained will have a crystal size of about a micron or a little less. To obtain smaller crystals, it is necessary to use smaller seeds. It is apparent therefore that there is a need for alpha alumina seed particles that are nano-sized so as to drive down the microcrystallite size of seeded sol-gel aluminas and yield the optimum products available from this technology.
The use of fine alpha alumina powder is also important in the production of formed ceramic articles or monoliths. In such a process a fine alumina powder is heated until the particles sinter together and form a solid body. This can be done by heating the powder compressed into the desired form under pressure, as in a hot isostatic pressure, (or HIP), operation or simply by heating a cold pressed powder in the form of the desired object. Clearly the smaller the powder particles the easier the sintering process. Thus in this field too there is a demand for powder that is as fine as possible.
Besides being used as a material from which ceramic monoliths or abrasive grits are formed, fine alpha alumina powder is widely used as a polishing, or lapping abrasive. In such lapping applications, the finer and more uniform the particle size of the powder, the better the finish that can be attained. Fine alpha alumina powder is also used to modify the frictional characteristics of materials as diverse as magnetic tapes and cardboard. In most of such applications, especially those where uniformity and fineness are desirable, nano-sized alpha alumina powder would be a highly desirable commodity.
Another significant market for nano-sized alumina is in the formulation of catalyst supports for high temperature catalytic operations.
One of the problems in working with a boehmite gel to produce formed ceramic articles is that the gel cannot exceed about 65 wt % solids because of the porous nature of the boehmite particles. Thus there is a lot of water that needs to be driven off in the course of the drying process. In addition not only is there further shrinkage as a result of the elimination of the water associated with the boehmite, (which is of course alpha alumina monohydrate), but the phase change from the intermediate gamma phase (to which the boehmite first converts) to the final alpha phase also involves a shrinkage. Thus the direct fabrication of a ceramic product from boehmite is only practical for thin objects where the water loss can be relatively easily be accommodated and the shrinkages can be controlled.
If alpha alumina in very fine form could be formed into a gel, it would be possible to form objects from the gel and then fire them to eliminate only the water associated with the gel without concern for the volume changes that would accompany changes of phase. The production of nano-sized alpha alumina poeders would make this objective a feasible proposition.
There is therefore a need to develop techniques for producing extremely fine alpha alumina powders that do not involve huge energy expenditures for crushing operations and which open up a wide range of potential new uses for such products.
The present invention provides a process that is adapted to provide alpha alumina in an extremely fine form that is very useful in a wide range of applications. The process is much more economical than prior art techniques and results in a much finer product than hitherto available that is of great versatility and value.
The invention also provides alpha alumina with a very uniform particle size in the nanometer range with a wide spectrum of potential applications.